Different turbine engine architectures make use of a propeller (turboprop, open rotor) or a variable-pitch fan. This variability allows the turbine engine to adapt itself to variable flight conditions by maintaining a favorable air incidence angle on the blades. The variability of pitch settings is particularly necessary for rotors having a low-pressure ratio, like turboprop propellers and turbine engine fans having a high bypass ratio (ratio of the secondary flow rate and the primary flow rate (that passes through the primary body)).
Multiple devices have been envisioned to vary the pitch of the blades. These devices generally comprise rotation of the blade around its main axis by bevel gears, located below the root of the blade. The latter cooperate with bevel gears of a control system configured to control the pitch of the blade.
In normal use, during flight phases, the possible interval for the pitch of a blade is generally sixty degrees or so, between a low pitch position with low forward speed and a high pitch position with high speed. Pitch settings in these normal operating conditions are, conventionally, called positive.
Shown in FIG. 3 is a sectional view of an example of a blade of a rotor (of a fan or of a turboprop propeller) on which the center of gravity G of the blade has been illustrated. It will be understood that, in the event of a failure, the blade has a tendency to rotate around the pivoting axis Y (in the direction of the arrow A) under the influence of centrifugal forces Fc applied to it. The blade then finds itself in a so-called “flat” position in which the blades form an angle of about 0° with the plane of rotation of the blades (i.e. the plane comprising the pivoting axes of the blades of the rotor, which is perpendicular to the axis of rotation of the rotor), which is very unfavorable to the airplane's drag. In fact, in this position, the drag of the rotor is a maximum and can put the aircraft in jeopardy by reducing the lift/drag ratio of the aircraft, which makes it difficult to continue flying with the remaining engine(s) by generating excessive drag, by creating an over-speed hazard and/or by generating strong asymmetry between the drags of the turbine engines of the aircraft, in the event that only one of these turbine engines has failed (which can make control of the airplane impossible).
One of the constraints on systems for controlling the pitch of the blades is therefore to bring them into a position called “feathered” in the event of a failure of the pitch change mechanism of the blade. The feathered position corresponds to a pitch greater than high pitch, forming an angle of roughly 90° with the plane of rotation of the blades. In this position, the chord of the blades is aligned substantially with the relative wind, thus reducing the drag that they generate and consequently the yaw imbalance produced on the aircraft. This feathered position also allows a reduction in the residual rotation speed of the blades.
In order to avoid having the blades move to the “flat” position in flight, the blades are generally put into the feathered position in the event of failure of the pitch change mechanism.
To this end, it has been proposed to use a system of eccentric high-density (approximately 19 tons per cubic meter) counterweights subjected to the centrifugal force and the inertia whereof, much greater than that of the blades, ensures the return of the latter into the feathered position when the pitch change mechanism fails. Ten to twelve in number, angularly distributed, these eccentric counterweights can total, for themselves alone, 150 to 200 kg. One can in particular refer to document FR2957329 in the Applicant's name for more details on this type of counterweighted system. This solution can, however, be penalizing in terms of mass because it requires the use of heavy counterweights whose force is not compounded. Moreover, this system adds considerable weight to the turbine engine, which increases its specific fuel consumption.
Also proposed in document WO 2012/066240 in the Applicant's name is to attach flyweights to the bevel gears of the blade pitch control mechanism so as to position them in cantilever with respect to them. The system is integrated into the spaces located between the blades for minimum use of axial and/or radial space. In normal operation, the flyweights are held in position by the pitch control system. In the event of failure of the pitch change mechanism, the action of the centrifugal force due to rotation of the propeller drives the flyweights toward a rest position which corresponds to the feathered position of the blade. According to the embodiment mentioned in this application, the bevel gear of the blade root gives rise to a reduction ratio of approximately two between the angle of the counterweight and the pitch angle of the blade. This solution therefore allows a reduction in the mass employed due to the compounding of the effects of the flyweights. However, freedom of choice remains limited due to constraints connected to the use of space of this system with respect to the available space. These constraints can therefore lead to preventing the integration of the system into the rotor (propeller or fan).